ML20275A102

Proposed Alternative I4r-07 to the Requirements of the American Society of Mechanical Engineers Code

ML20275A102
Person / Time
Site:	Calvert Cliffs
Issue date:	11/04/2020
From:	Nancy Salgado Plant Licensing Branch 1
To:	Bryan Hanson Exelon Generation Co, Exelon Nuclear
Wiebe J
References
EPID L-2019-LLR-0118
Download: ML20275A102 (7)

Text

November 4, 2020 Mr. Bryan C. Hanson Senior Vice President Exelon Generation Company, LLC President and Chief Nuclear Officer (CNO)

Exelon Nuclear 4300 Winfield Road Warrenville, IL 60555

SUBJECT:

CLINTON POWER STATION, UNIT 1 - PROPOSED ALTERNATIVE I4R-07 TO THE REQUIREMENTS OF THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS CODE (EPID L-2019-LLR-0118)

Dear Mr. Hanson:

By letter dated December 16, 2019 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML19350C642), as supplemented by letter dated August 13, 2020 (ADAMS Accession No. ML20226A443), Exelon Generation Company, LLC (the licensee) submitted a request to the U.S. Nuclear Regulatory Commission (NRC) for the use of an alternative to certain American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI, requirements at Clinton Power Station, Unit 1 (Clinton).

Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(z)(1), the licensee requested to use alternative I4R-071 on the basis the alternative provides an acceptable level of quality and safety. The licensee proposed to eliminate the volumetric examination of the reactor pressure vessel threads in flange (Category B-G-1 examinations) during the fourth inservice inspection (ISI) interval at Clinton.

The NRC staff has reviewed the subject request and concludes, as set forth in the enclosed safety evaluation, that the licensees proposed alternative provides an acceptable level of quality and safety. Accordingly, the NRC staff concludes that the licensee has adequately addressed all of the regulatory requirements set forth in 10 CFR 50.55a(z)(1). Therefore, the NRC staff authorizes the use of the proposed alternative in request No. I4R-07 at Clinton for the duration of the fourth 10-year ISI interval, which commenced on July 1, 2020, and is currently scheduled to end June 30, 2030.

All other ASME Code,Section XI, requirements for which relief was not specifically requested and approved remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.

1 The other requests contained in the licensees December 16, 2019, letter have been addressed by separate correspondence.

B. Hanson If you have any questions, please contact the Senior Project Manager, Joel S. Wiebe, at (301) 415-6606 or Joel.Wiebe@nrc.gov.

Sincerely, Digitally signed by Robert F. Robert F. Kuntz Date: 2020.11.04 Kuntz 09:50:33 -05'00' Nancy L. Salgado, Chief Plant Licensing Branch III Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-461

Enclosure:

Safety Evaluation cc: Listserv

SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION PROPOSED ALTERNATIVE I4R-07 REGARDING VOLUMETRIC EXAMINATION OF REACTOR PRESSURE VESSEL THREADS IN FLANGE EXELON GENERATION COMPANY, LLC CLINTON POWER STATION, UNIT 1 DOCKET NO. 50-461

1.0 INTRODUCTION

By letter dated December 16, 2019 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML19350C642), as supplemented by letter dated August 13, 2020 (ADAMS Accession No. ML20226A443), Exelon Generation Company, LLC (EGC, the licensee) requested an alternative from the requirements of the American Society of the Mechanical Engineers Boiler and Pressure Vessel (ASME Code) for Clinton Power Station, Unit 1 (Clinton).

The EGC proposed alternative, I4R-07, proposes to eliminate the volumetric examination of the reactor pressure vessel (RPV) threads in flange (Category B-G-1 examinations) during the fourth inservice inspection (ISI) interval at Clinton.

Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR) Section 50.55a(z)(1), EGC requested to use the proposed alternative on the basis it provides an acceptable level of quality and safety.

2.0 REGULATORY EVALUATION

The regulations in 10 CFR 50.55a(g)(4) mandate that ASME Code Class 1, 2, and 3, components meet the ISI examination requirements set forth in ASME Code Section XI. The ISI examinations are to comply with the requirements in the latest edition and addenda of Section XI Code incorporated by reference in 10 CFR 50.55a(a), subject to the limitations and modifications listed in 10 CFR 50.55a(b). The RPV threads in flange are ASME Code Class 1 components. Therefore, per 10 CFR 50.55a(g)(4), ISI of these threads must be performed in accordance with Section XI of the applicable edition and addenda of the ASME Code.

The regulations in 10 CFR 50.55a(z) state:

Alternatives to the requirements of paragraphs (b) through (h) of this Section

[50.55a] or portions thereof may be used when authorized by the Director, Office of Nuclear Reactor Regulation. A proposed alternative must be submitted and Enclosure

authorized prior to implementation. The applicant or licensee must demonstrate that:

(1) Acceptable Level of Quality and Safety. The proposed alternative would provide an acceptable level of quality and safety; or (2) Hardship without a Compensating Increase in Quality and Safety.

Compliance with the specified requirements of this Section [50.55a] would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.

Based on the above, and subject to the following technical evaluation, the NRC staff finds that the licensee may propose an alternative to ASME Code,Section XI, and the NRC staff has the regulatory authority to authorize the licensee's proposed alternative.

3.0 TECHNICAL EVALUATION

3.1 Licensees Request for Alternative 3.1.1 ASME Code Components Affected Proposed alternative I4R-07 applies to the RPV threads in flange, Examination Category B-G-1, Item No. B6.40, in Section XI of the ASME Code.

Examination Examination Code Category Item No. Method Description Class B-G-1 B6.40 Volumetric RPV Threads in Flange 1 3.1.2 Applicable Code Edition and Addenda The code of record for the fourth ISI interval at Clinton, for the inspection of ASME Code Class 1, 2, and 3, components is the ASME Code,Section XI, 2013 Edition.

3.1.3 Applicable ASME Code Requirements The ASME Code,Section XI, Table IWB-2500-1, Examination Category B-G-1, Pressure Retaining Bolting, Greater Than 2 in. (50 mm) in Diameter, requires volumetric examination of the RPV threads in flange. The required examination area of 1-inch around each RPV stud hole is depicted in ASME Code,Section XI, Figure IWB-2500-12, Closure Stud and Threads in Flange Stud Hole.

3.1.4 Description of Proposed Alternative I4R-07 The proposed alternative I4R-07 applies to the requirements in ASME Code,Section XI, Table IWB-2500-1. The proposed alternative would eliminate the volumetric examination of Category B-G-1 RPV threads in flange during the fourth ISI interval at Clinton, or until the NRC approves ASME Code,Section XI, Code Case N-8642, "Reactor Vessel Threads in Flange Examination,"

Section XI, Division 1. The fourth ISI interval for Clinton commenced on July 1, 2020, and is scheduled to end on June 30, 2030.

2 ASME Section XI Code Case N-864 is available at https://www.asme.org/.

3.1.5 Licensees Technical Basis for Proposed Alternative I4R-07 In its December 16, 2019, letter, Attachment 7, Section 5, Proposed Alternative and Basis for Use, the licensee states that the technical basis for eliminating the RPV threads in flange volumetric examinations is provided in Electric Power Research Institute (EPRI) Technical Update Report, Nondestructive Evaluation: Reactor Pressure Vessel Threads in Flange Examination Requirements (EPRI Report No. 3002007626) (ADAMS Accession No. ML16221A068).

The licensee also states in its December 16, 2019, letter that the requirements in the relief request are based on ASME Code Case N-864, which has been approved by the ASME Board on Nuclear Codes and Standards but has not been incorporated into 10 CFR 50.55a.

Additionally, the licensee states that Clinton uses detailed procedures for the care and visual inspection of the RPV threads in flange and studs every time the head is removed.

In its letter dated December 16, 2019, the licensee provided a description of the maintenance activities and inspections to be performed on the RPV threads in flange and studs each time the RPV head is removed during the fourth ISI interval. Descriptions of the site-specific procedures controlling these activities were also included. The licensee cites its corrective action program for addressing degradation of the RPV threads in flange, if detected.

In its letter dated December 16, 2019, Attachment 7, Section 4, Reason for Request, the licensee discusses the technical information provided in EPRI Report No. 3002007626, which is cited as the basis for proposed alterative I4R-07. Section 4 discusses operating experience (OE) related to the RPV threads in the flange, potential degradation mechanisms, stress analysis, flaw tolerance evaluation, and other related RPV assessments. The licensee notes that the conclusion from EPRI Report No. 3002007626, is that the current requirements are not commensurate with the associated burden (worker exposure, personnel safety, radwaste, critical path time, and additional time at reduced water inventory) of the examination.

3.2 NRC Staff Evaluation Proposed alternative I4R-07 is based upon ASME Code Case N-864. Code Case N-864 has not been incorporated in Regulatory Guide 1.147, Revision 19, Inservice Inspection Code Case Acceptability, ASME Code,Section XI, Division 1, dated October 2019 (ADAMS Accession No. ML19128A244). As a result, this code case is not available for application without specific NRC approval. The below evaluation provides the basis for specific approval for use at Clinton.

The technical basis for proposed alternative I4R-07 is provided in EPRI Report No.

3002007626. The NRC staff reviewed the industry's generic stress analysis and flaw tolerance evaluation documented in Section 6 of the report and determined it to be acceptable based on the NRC evaluation in Section 3.2 of the safety evaluation transmitted by letter dated January 26, 2017 (ADAMS Accession No. ML17006A109). The NRC staff reviewed the Clinton-specific information provided in the licensees letter dated December 16, 2019, and concluded that the EPRI analysis and the NRC staffs evaluation in its letter dated January 26, 2017, are applicable to Clinton because its RPV flange is made of similar materials. Therefore, the NRC staff focused its evaluation of proposed alternative I4R-07 on aspects related to maintaining reasonable assurance of adequate defense-in-depth. In particular the staff were focused on the effect of not conducting volumetric inspections on the RPV threads in flange for a second sequential ISI interval.

The NRC staff reviewed the licensees description of the maintenance and inspection activities associated with the RPV threads in flange and studs during the fourth ISI interval at Clinton.

The objective of this review was to determine if the maintenance activates and inspections provide reasonable assurance that unforeseen degradation of the RPV threads in flange will be detected and mitigated prior to returning the reactor to service each time the RPV head is removed during the fourth ISI interval.

Based on the information in the licensees letter dated December 16, 2019, the NRC staff determined that each time the RPV head is removed all studs are individually cleaned, which allows access to all the RPV threads in flange on a periodic basis (i.e., each refueling outage).

The licensee stated that once the studs are removed the RPV flange and stud threads are inspected and cleaned.

Based on the information in the licensees letter dated December 16, 2019, the NRC staff determined that maintenance and inspection activities are also performed on each RPV stud, nut, and washer each time the RPV head is removed. The condition of the mating threads and adjacent RPV closure stud components provides additional information on the condition of the RPV threads in flange. The plant-specific procedures verify that all RPV stud and nut threads have been inspected for nicks, marks, or signs of galling. During reactor assembly the head flange, stud holes, stud washers, closure nuts, and removed studs are cleaned again.

In summary, the NRC staff determined that the stress analysis, flaw tolerance evaluation, and OE review in the generic EPRI Report No. 3002007626 evaluation are bounding for Clinton.

The NRC staff further determined that the combination of maintenance and inspection activities of the RPV threads in flange and associated RPV closure stud components provide reasonable assurance that unforeseen degradation of the RPV threads in flange will be detected and mitigated prior to returning the reactor to service each time the RPV head is removed during the fourth ISI interval. Therefore, there is reasonable assurance that adequate defense-in-depth will be maintained without the volumetric inspections. Based on the above, the NRC staff finds that alternative I4R-07 provides an acceptable level of quality and safety.

4.0 CONCLUSION

As set forth above, the NRC staff finds that proposed alternative I4R-07 provides an acceptable level of quality and safety. Accordingly, the staff concludes that the licensee has adequately addressed all of the regulatory requirements set forth in 10 CFR 50.55a(z)(1). Therefore, the NRC staff authorizes the use of relief request I4R-07 for the fourth ISI interval at Clinton, which commenced on July 1, 2020 and is scheduled to end on June 30, 2030.

All other ASME Code,Section XI, requirements for which relief was not specifically requested and approved remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.

Principal Contributor: D. Widrevitz Date: November 4, 2020

ML20275A102 *via email OFFICE NRR/DORL/LPL3/PM NRR/DORL/LPL3/LA* NRR/DNRL/NVIB/BC* NRR/DORL/LPL3/BC*

NAME JWiebe SRohrer HGonzalez NSalgado (R. Kuntz for)

DATE 10/7/20 10/ 6/20 9/11/20 11/4/20